Electric motor drive systems, which include power electronic inverters, commonly are used to produce and control the phase currents supplied to electric motors. For example, power electronic inverters may be used to control the torque and speed output of alternating current (AC) motors in electric vehicles and hybrid electric vehicles (HEVs). Accurate determination of the phase currents assists in achieving high performance torque regulation. Accordingly, in a digitally-controlled system, high performance current sensors (e.g., analog-to-digital (A-to-D) converters) often are used to obtain measurements of the phase currents. In a three-phase system, only two of the phase currents are independent quantities. Accordingly, it is common only to measure two of the phase currents and to derive the third phase current. For this reason, traditional electric motor drive systems utilize two current sensors to obtain phase current measurements, and values for the third phase current are calculated, as will be described in more detail below.
Because the sensed phase currents may have an offset component, some traditional systems take an initial measurement of the phase currents prior to starting the motor drive system's operations. The measurement may be stored as an initial offset current, ioffset, which indicates the initial current conditions of the current sensors. During operations, an approximation of the AC portion of a phase current, icontrol, may be calculated by subtracting the offset current from the measured current, imeasured, as shown in Equation 1 (Equ. 1):icontrol=imeasured−ioffset  Equ. 1
The phase current approximation, icontrol, is the quantity that is analyzed in performing torque regulation. The phase current approximation technique given by Equ. 1 yields a representation of the motor phase currents that may facilitate high performance torque and speed control in some situations. However, using Equ. 1, the accuracy of the approximation of icontrol may be compromised when ioffset changes significantly during operations. Such changes may occur, for example, due to significant changes in the system operating conditions, such as substantial changes in ambient operating temperature conditions from the conditions that were initially present. Substantial ambient operating temperature variations commonly occur during a motor vehicle driving cycle, for example, and these temperature variations may affect the current sensor measurements and, thus, the actual offset current. In other words, the actual offset current may “drift” during operations, from the value of the initial offset current, ioffset. The phenomenon of offset current drift is commonly experienced, for example, by open loop types of current sensors, although closed loop types of current sensors also may experience offset current drift.
Torque control of three-phase AC motors may be based on a two-axis model of the electric motor. Because only two of the phase currents are independent quantities in a three-phase system, as mentioned above, it is common only to measure two of the phase currents, and to derive the third phase current. For example, if the phase a current, ia, and the phase b current, ib, are measured, the phase c current, ic, may be derived according to Equ. 2 as:ic=−(ia+ib)  Equ. 2
The two-axis, reference frame currents, iα and iβ, may be calculates according to Equ. 3 as:
                              [                                                                      i                  β                                                                                                      i                  α                                                              ]                =                  2          /                                    3              ⁡                              [                                                                            0                                                                                                                3                                                2                                                                                                                                      -                                                      3                                                                          2                                                                                                                        1                                                                                                                -                          1                                                2                                                                                                                                      -                          1                                                2                                                                                            ]                                      ⁡                          [                                                                                          i                      a                                                                                                                                  i                      b                                                                                                                                  i                      c                                                                                  ]                                                          Equ        .                                  ⁢        3            And the synchronous frame currents, id and iq, may be calculated according to Equ. 4 as:
                              [                                                                      i                  d                                                                                                      i                  q                                                              ]                =                              [                                                                                cos                    ⁢                                                                                  ⁢                                          θ                      e                                                                                                            sin                    ⁢                                                                                  ⁢                                          θ                      e                                                                                                                                                              -                      sin                                        ⁢                                                                                  ⁢                                          θ                      e                                                                                                            cos                    ⁢                                                                                  ⁢                                          θ                      e                                                                                            ]                    ⁡                      [                                                                                i                    α                                                                                                                    i                    β                                                                        ]                                              Equ        .                                  ⁢        4            
FIG. 1 is a chart illustrating three, sinusoidal phase currents 102, 103, 104. Axis 106 represents electrical position, in degrees, and axis 108 represents amplitude. Phase current 102 may represent a measured phase a current, ia, phase current 103 may represent a measured phase b current, ib, and phase current 104 may represent a calculated phase c current, ic. In FIG. 1, about a 5% positive offset drift is shown to be present in the measured values for the phase a and b currents, 102, 103, as indicated by the upwardly shifted position of the phase a and b currents 102, 103. Because the phase c current 104 is calculated based on the measured phase a and b currents 102, 103 (e.g., using Equ. 1 and 2, above), the phase c current 104 reflects about a 10% negative offset drift, as indicated by the downwardly shifted position of the phase c current 104. This downward shift affects the calculation of the reference frame currents, iα and iβ, as is illustrated in FIG. 2, which in turn affects the calculation of the synchronous frame currents, id and iq, as is illustrated in FIG. 3.
FIG. 2 is a chart illustrating reference frame currents 202, 203, iα and iβ, respectively, which are calculated based on the measured and calculated phase currents 102-104 illustrated in FIG. 1 (e.g., using Equ. 3, above). As FIG. 2 illustrates, the reference frame currents 202, 203 include a wobble, as indicated by reference frame current 203 being slightly elevated, with respect to reference frame current 202. FIG. 3 is a chart illustrating synchronous frame currents 302, 303, id and iq, respectively, which are calculated based on the reference frame currents 202, 203 illustrated in FIG. 2 (e.g., using Equ. 4, above). For an ideal system without any offset drift, the synchronous frame currents, id and iq, would be purely direct current (DC) quantities. However, as FIG. 3 illustrates, the synchronous frame currents 302, 303 include a significant non-fundamental DC component.
A high performance, current regulated, electric motor drive system attempts to regulate the synchronous frame currents to DC quantities. In traditional systems, the presence of an offset drift will have the effect of producing a non-fundamental component in the synchronous frame currents (e.g., as illustrated in FIG. 3), which will be reflected back into the actual phase currents. Any non-fundamental component in the actual phase currents produces an undesirable torque ripple, which may adversely affect system performance.
High performance drives benefit from precise sampling determination of motor phase currents for purposes of torque control, even in the face of significant offset drifts, which may occur during operations. Accordingly, it is desirable to provide apparatus and methods to compensate adequately for offset drifts that may detrimentally affect the phase currents, and thus the performance, of an electric motor. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.